1. Field of the Invention
This invention relates to photo-polymerizable compositions and to thermally stable photopolymer compositions prepared therefrom. Another aspect of the invention relates to light transmissive optical devices, such as waveguides formed from the photopolymer composition of this invention.
2. Prior Art
In optical communication systems, messages are transmitted by carrier waves of optical frequencies that are generated by light sources such as lasers or light-emitting diodes. There is much current interest in such optical communication systems because they offer several advantages over conventional communication systems, such as a greatly increased number of channels of communication and the ability to use other materials besides expensive copper cables for transmitting messages. One such means for conducting or guiding waves of optical frequencies from one point to another is called an "optical waveguide." The operation of an optical waveguide is based on the fact that when a medium which is transparent to light is surrounded or otherwise bounded by another medium having a lower refractive index, then light introduced along the inner medium's axis is highly reflected at the boundary with the surrounding medium, thus producing a guiding effect.
The most frequently used material for such a waveguide device is glass, which is formed into a fiber of specified dimension. However, optically-transparent polymers are increasingly being used as the light transmissive element. Conventional optically-transparent polymers suffer from one or more defects. For example, a common property of optically transparent polymers is that the polymers become discolored when heated for extended periods of time. The visual effect is that the polymers become yellow. The thermally-induced discoloration in polymers results from a number of factors as for example chemical reactions that slowly degrade the polymer. Longer heating times usually cause greater discoloration than shorter heating times. What initially might be a light yellow discoloration may eventually turn to brown or even black. If a sample of discolored polymer is examined in an absorption spectrophotometer, an absorption band tail can be observed that extends from the intrinsic ultraviolet absorption bands of the polymer into the visible spectral region (400 nm to 700 nm) and even into the infrared spectral region (&gt;700 nm). The absorption tail typically has no discernable fine structure. The absorption of light in the region of transmission reduces the intensity of the optical wave as it passes through the waveguide, a phenonemon which contributes to "optical loss" or "optical power loss".
Multimode optical waveguides are used primarily in the infrared spectral region from 700 nm to 1300 nm. If a polymer optical waveguide is heated for an extended period of time and if a thermally-induced absorption band tail extends into this infrared wavelength region of operation, the transmission of light through the waveguide can be dramatically reduced. For cases of severe discoloration, the waveguide becomes useless.
Photopolymer films and waveguides can crack when heated to elevated temperatures. A crack in a waveguide scatters light and generally reduces the effectiveness of the waveguide. Such cracking is a result of stress. The stress can be internal to the waveguide, resulting from non-uniformities in the photopolymer composition, or stress can result from substrate or overcoat materials in contact with the waveguide which have different coefficients of thermal expansion than the waveguide material. Even the process of forming a photopolymer waveguide on a substrate by UV exposure of monomer precursors generally results in a waveguide that is under tensile stress. This occurs because the volume occupied by a photopolymer is usually smaller than the volume of the monomer precusors. The amount of shrinkage can be as much as 10%.
A third effect which results from changing the temperature is delamination of the waveguide from the substrate. Delamination changes the properties of the waveguide (e.g. the numerical aperture or NA) and weakens the waveguide structure so that it is more susceptible to damage from vibration and other environmental factors such as moisture.